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Abstract
The modifications of multilayer dielectric (MLD) gratings arising from laser-induced damage using 0.6-ps and 10-ps laser pulses at 1053 nm are investigated to better understand the damage-initiation mechanisms. Upon damage initiation, sections of the affected grating pillars are removed, thereby erasing the signature of the underlying mechanisms of laser damage. To address this issue, we performed paired studies using macroscopic grating-like features that are 5 mm in width to reveal the laser-damage morphology of the different grating sections: pillar side wall, sole, and pillar top. The results suggest that, similarly to MLD coatings, there are two damage-initiation mechanisms corresponding to the different pulse durations.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The proliferation in recent years of high-peak-intensity laser systems around the world is motivated by opportunities to explore new physical phenomena under “strong-field” excitation with short-pulse lasers [1–3]. This was enabled by the development of suitable laser technologies, first demonstrated by Gérard Mourou and Donna Strickland in 1985 [4]. Because direct amplification of ultrashort laser pulses is not possible as a result of nonlinear effects, the laser pulse is first temporally stretched before it undergoes amplification and subsequent recompression. This process is achieved using diffraction gratings. The resistance of the pulse-compression gratings to laser-induced damage limits the energy output, which affects the operational cost associated with refurbishment or replacement of optics. Developing the underlying technological breakthroughs to increase the LIDT of the constituent optical elements for such systems will facilitate broader availability to the scientific community for basic research.
The first generation of diffraction gratings was based on a metal-coated microstructured surface that, because of absorption, provided limited energy output and reduced laser-induced-damage threshold (LIDT). The second generation of gratings was based on multilayer dielectric (MLD) coatings [5,6], which exhibited significantly lower localized absorption and higher LIDT. A hybrid version has also been developed where the metal coating is deposited under a number of dielectric coating layers to reduce the laser power reaching the metal while taking advantage of the larger bandwidth that it can support [7,8]. In MLD and hybrid gratings, the top layer is processed, using a photolithography method, to contain a 2-D structure comprised of submicrometer-sized linear pillars and corresponding trenches (also referred to as “soles”). that extend over the length of the grating, which can be of the order of 1 m in size for the highest-energy, short-pulse laser systems.
The laser-damage resistance of current-generation MLD gratings is limited by the presence of two general classes of defects imbedded in the coating structure during the manufacturing process [9] or subsequent handling and exposure to the operational environment [10]. The first class of defects is incorporated during the coating-deposition process, consisting of atomic defects providing absorption states inside the band gap or nanoscale defect structures. This class of defects is also responsible for lowering the damage performance in other types of optical elements that are based on MLD coatings such as transport mirrors, polarizers, etc. The second class of defects is associated with the photolithography and etching process used to generate the grating pillars and is most likely associated with pillar distortions and nonstoichiometric contaminants [11,12]. This is evident from that fact that the LIDT in gratings is lower than predictions based on the LIDT of the MLD coating and the electric-field enhancement from a perfect grating structure. Previous studies have demonstrated that damage (at threshold conditions) initiates on the pillars at the location of peak electric-field intensity [13,14]. There are no reports that examine the exact mechanism of energy deposition and its relation to either pre-existing defects in the coating or defects associated with the photolithography and etching process used to generate the pillars. In addition, there are no studies that examine the damage mechanism in gratings as a function of the laser pulse duration.
In this work, we investigate the laser-damage mechanisms in MLD gratings by correlating the damage morphology in actual gratings to those observed within the different sections (pillar side wall, sole, and pillar top) of wider, mechanically stable grating-like structures. Furthermore, these macroscopic grating-like structures (which have a width of 5 mm) generate an internal electric-field enhancement that approximates that generated at the pillar surface of actual gratings. The study was performed using a laser system operating at 1053 nm and pulse durations of 0.6 ps and 10 ps. The results suggest that the damage-initiation mechanisms are analogous to those governing damage in MLD coatings involving two distinct mechanisms that are characteristic of different pulse-duration regimes. The results may further suggest that damage is driven by coating defects, but the presence of contamination-induced defects introduced during the photolithography and etching process act synergistically, therefore decreasing the overall damage threshold.
2. Experimental arrangement
The millimeter-pitch grating-like structures were etched into an 800-nm silica (SiO2) layer (deposited using electron-beam evaporation) on 100-mm-diam, 3-mm-thick fused-silica substrates. A photoresist and a bottom antireflective coating (BARC) layer were used to create the etching mask using photolithographic processes. The mask pattern was then transferred to the SiO2 coating layer via reactive ion etching, creating the 5-mm-wide pillars and soles in the SiO2 coating layer. The width of the features was chosen to allow spatially targeted damage testing of the different regions (i.e., the pillar side wall, pillar top, and sole). The height of the pillar was 700 nm [measured with atomic force microscopy (AFM)], which is similar to the height of the pillars in MLD gratings (see depiction in Fig. 1). The substrates were broken into 1/4 and 1/8 pie-sections and were cleaned to remove the remnants of the manufacturing process using cleaning processes based on work by Howard et al [11]. In addition, the electric-field–intensity (EFI) enhancement near the wall region for the irradiation geometry used in this work (61° angle of incidence) is analogous to that occurring within the pillar region in MLD gratings. Specifically, the spatial distribution of the EFI enhancement shown in Fig. 2(a), which is estimated using a coherent ray-tracing code implemented in Interactive Data Language (IDL), indicates an area of increased EFI of up to threefold at a depth of 250 to 500 nm (from the surface of the pillars) and at a distance of ∼225 to 790 nm from the surface of the wall.
Fig. 2. The electric-field–intensity (EFI) enhancement (a) near the millimeter-pitch pillar wall region and (b) for the multilayer dielectric (MLD) grating design used in this work. The laser irradiation is incident on the samples at a 61° angle from the left side. The laser beam is s-polarized in both cases (parallel to the grating ridges)
Small-size MLD gratings were used for the damage-testing experiments. The MLD coatings were a modified quarter-wave, thin-film high reflector using SiO2 and hafnia (HfO2) as the low- and high-refractive-index materials, respectively. The coating was fabricated via ion-assisted electron-beam evaporation on 3-mm-thick, 100-mm-diam fused-silica substrates. The grating structure was created using interference lithography and reactive ion etching to pattern a 1740-lines/mm structure in the top silica layer of the MLD coating. The EFI enhancement for the grating was modeled with KAPPA [15] using the irradiation geometry tested for this work. The intensity enhancement is shown on the pillar wall that is opposite the incoming laser beam. The spatial distribution of the EFI enhancement is shown in Fig. 2(b) with a 3× intensity enhancement near the pillar wall.
Damage testing was performed using a laser system operating at 1053 nm and having a tunable pulse duration between 0.6 ps and 100 ps. This system has been described in detail elsewhere [9,11,16]. The samples were tested in a vacuum (4 × 10–7 Torr) environment with s-polarized light at a 61° angle of incidence. The results presented in this work were obtained using pulse lengths of 0.6 and 10 ps that support two distinct damage-initiation mechanisms (detailed in the next section). The 1/e laser beam diameter on the sample (normal to direction of beam propagation) was ∼350 µm. Damage testing was performed in a single-shot (1-on-1) regime, and damage thresholds were determined for each specific case using in-situ optical microscopy. Subsequently, damage sites were characterized using Nomarski differential interference contrast microscopy, AFM, and scanning electron microscopy (SEM).
3. Experimental results
Figures 3 and 4 show SEM images of laser-induced–damage sites in MLD grating samples under exposure to 0.6-ps (Fig. 3) and 10-ps (Fig. 4) pulses. These damage sites were generated at 1.1 J/cm2 for the 0.6-ps pulses and 2.2 J/cm2 for the 10-ps pulses. The laser beam irradiates the grating sample from the left side as viewed in the SEM images shown in both figures. In both cases, the damage morphology encompasses the removal of pillar sections in localized areas as seen in Fig. 3(a) and 4(a), which in turn conceals the signatures of the damage mechanism. Removal of individual pillar sections is observed under 0.6-ps excitation, mainly on the right side of the pillars where the EFI enhancement is a maximum. The missing sections exhibit smooth surfaces in the regions where material was removed which arguably may arise from fracturing and/or melting of small sections of individual pillars. In addition, nanoparticle debris is observed in the adjacent sole regions as exemplified in the contrast-enhanced SEM image shown in Fig. 3(b).
Fig. 3. SEM images of areas of an MLD grating where (a) laser-induced damage was initiated under a single 0.6-ps laser pulse and (b) the image of the central region at a higher magnification shows nanoparticle distribution. The laser irradiation impinges on the samples at a 61° angle of incidence from the left side.
Fig. 4. SEM images of areas of MLD grating where (a) laser-induced damage was initiated under a single 10-ps laser pulse and (b) a single damage site under a higher magnification (showing nanoparticle distribution). The laser irradiation impinges on the samples at a 61° angle of incidence from the left side.
Damage initiation under excitation with 10-ps pulses (see examples in Fig. 4) also exhibits removal of pillar sections, but there are characteristic differences compared to the morphology of damage sites formed with 0.6-ps pulses. Specifically, the sections of missing pillars, are significantly larger, often reaching the base of the pillars. Also, damage sites typically encompass multiple pillars, and a larger collection of debris is observed in the adjacent sole regions including flakes of mechanically damaged pillar sections and molten nanoparticle debris as exemplified in the contrast-enhanced SEM image shown in Fig. 4(b).
Damage testing was performed on each region of the five millimeter-pitch grating-like samples (pillar tops, sole, and pillar walls) used in this work. An unprocessed silica monolayer was also tested for comparison to the photolithography-processed samples with grating-like structures. Damage testing for all samples was performed with a single-shot method (1-on-1) to determine the LIDT using 61° angle of incidence. Testing of the specific individual regions (pillar, sole, and wall) was straightforward, due to the fact that a) these features were visible in the online microscope system, b) the soles are distinguishable from the pillars as they exhibit slightly higher scattering and c) the beam is much smaller than the width of the pillars and soles. The damage testing results are summarized in Table 1, where the values shown represent the average LIDT value from the five samples tested for this work. The values from the sole and pillar top are slightly lower than the values obtained for the control silica coating. In contrast, the damage threshold at the wall region was about fourfold lower at 0.6 ps and about twofold lower at 10 ps.
Table 1. The average laser-induced–damage threshold (LIDT) for the five millimeter-pitch grating-like samples and control coating sample.
Figures 5 and 6 show SEM images of damage sites at the wall region under exposure to 0.6- and 10-ps pulses, respectively. The left inset in Fig. 5(a) shows a high-magnification image of an undamaged section of the wall region, where the sole and pillar-top regions are visible. The sole region has characteristic silica “grass” [17] features from the etching process creating nanoscale roughness in that area. Still, these features do not seem to considerably alter the damage threshold. The wall also exhibits roughness in the 100-nm (or smaller) spatial scale. The damage region under exposure to 0.6-ps pulses involves removal of the upper section of pillar material adjacent to the wall. The SEM image shown in the right inset in Fig. 5(a) shows in high magnification the morphology of the damage site. This morphology is reminiscent of the type-I damage morphology observed in MLD coatings. Specifically, the length of the damage site is related to the diameter of the peak intensity for the laser beam, while the depth of the damage site is related to the location and depth of the EFI enhancement [depicted in Fig. 2(a)] as revealed by the AFM lineout of the damage site shown in Fig. 5(b). Furthermore, the surface of the damage site in a grating-like structure contains the remains of melted-silica nanoscale projections and fibers that have solidified at the crater base and fracture edges. These features are identical to such features observed in type-I damage sites in MLD coatings [see Fig. 11(b) in Ref. 9] and are indicative of the separation of liquid nanodroplets during explosive boiling. This morphology has been attributed to phase instabilities during the laser-induced explosive-boiling process involved during the laser-damage event and is a typical characteristic in fused silica, even in laser-damage sites generated with nanosecond pulses [18,19].
Fig. 5. (a) SEM images of the wall region in millimeter-pitch grating-like structures containing a damage site generated with a 0.6-ps pulse. The left inset shows in higher magnification the undamaged part of the wall region and the right inset shows a section of the damage region. The laser beam impinges from the top at 61°. (b) A cross-sectional lineout of the damage region obtained with AFM imaging reveals the depth and exact position of the damage site.
Fig. 6. SEM images of different damage sites generated under 10-ps pulses in the wall region in millimeter-pitch gratings.
Typical examples of the damage site morphology in the wall region of the grating-like structure generated with 10-ps pulses is shown by the SEM images in Fig. 6. This morphology is fundamentally different than that obtained with a 0.6-ps pulse and contains distinct damage pits that are reminiscent of the type-II damage morphology observed in MLD coatings [9]. This morphology highlights the difference in the damage-initiation mechanism and indicates that damage initiation occurs at isolated defects near the location of peak EFI (as damage pits are located in the same area of maximum intensity enhancement). It has been previously discussed that the temperature and pressure relaxation pathways following plasma formation at the defect location govern the morphology of defect-driven type-II damage [9]. The energy absorbed causes localized heating of the surrounding material, while the generated pressure ultimately results in the formation of a venting pit, which can be accompanied by mechanical fracture of the surrounding areas. These features are clearly present in the damage sites of grating-like structures generated with 10-ps pulses.
4. Discussion
It must be noted that damage in MLD coatings exhibit three damage mechanisms and associated damage morphologies [9]. Type-I damage occurs at pulse widths <2.5 ps and is driven by the EFI distribution in the coating, causing volume breakdown of the material and the pressure-induced removal of the overlying coating material. Damage initiates from increased nonlinear absorption by defect and localized states located in the band gap [9,20]. This results in a shallow crater with fractured edges and remnant melted material at the base of the crater. Type-II and type-III damage morphologies are similar in that they occur at pulse widths >2.5 ps and are driven by isolated material defects (damage precursors). Damage initiation occurs near areas with high EFI for type-II damage and <150 nm from the coating surface for type III. The resulting morphology for both of these types has evidence of melting and boiling near the crater center with edges formed by mechanical failure.
Translating the results obtained using the millimeter-pitch grating-like structures (examples shown in Figs. 5 and 6) with those observed in actual MLD gratings (examples shown in Figs. 3 and 4), the two pulse-length regimes must be considered separately since the governing damage-initiation mechanism in each regime is different. Considering damage with 0.6-ps pulses, the type-I damage process (volume breakdown) is confined within the region of peak EFI, which is located on the right side near the top of the pillars in the image shown in Fig. 2(b). The extent of this region is less than the width of a single pillar. The superheating of this small material volume is manifested by the observation in the adjacent region of flakes [which appear to be fragments of pillars; see Fig. 3(a)] and round nanoparticles [such as those observed on the adjacent soles; in Fig. 3(b)], which are remnants of ejected liquid droplets. Owing to the small material volume involved, the overall energy deposited is insufficient (at the damage-threshold fluence) to remove all or even large sections of the pillar. The removal of large pillar sections is observed for exposure fluences greater than that required to initiate damage.
The 10-ps damage sites in MLD gratings are confined to isolated areas within the damage-testing laser beam footprint, indicative of defect-driven damage [see example in Fig. 4(a)]. Furthermore, damage (at threshold conditions) involves large-section removal of a single pillar or group of adjacent pillars. The debris within damage sites includes the deposition of nanoparticles as well as pillar fragments [see Figs. 4(a) and 4(b)]. Of particular interest is the nanoparticle distribution within the damage region. The contrast-enhanced SEM image in Fig. 4(b) indicates that the nanoparticles are confined in the sole region adjacent to the middle pillar of the damage site. This may be indicative that damage initiated in the middle pillar of this three-pillar damage site.
Experiments with millimeter-pitch grating samples show that damage is due to defects (type-II) generating damage pits that are on the same spatial scale as the pillar width in actual gratings. Assuming that such defect structures (which were estimated to have a 10- to 50-nm diameter) [9] are also initiating damage in gratings, we need to consider the following: (a) damage-initiating defects may be found at different depths within the pillars, therefore, the depth of damage initiation is not well defined; (b) the energy deposited will be sufficient to remove a large section of the main pillar; and (c) when damage initiates deep within a pillar, the partial confinement of the generated plume by the adjacent pillars may generate pressure that exceeds their mechanical strength. As a result, damage can involve (the removal of) multiple pillar sections. These general characteristics are observed in the damage morphology with 10-ps pulses and highlighted in the example shown in Fig. 4(b).
The damage-testing results shown in Table 1 for the millimeter-pitch grating-like structures delineate a number of interesting behaviors. First, the average LIDT at the sole sections is similar to that of the pillar-top LIDT for both tested pulse widths. The damage thresholds for both regions are slightly lower than that of the control coating for both pulse widths. This may indicate that the as-formed sole surface (although it contains “silica grass”) and the unetched pillar top are similarly modified during the fabrication and cleaning processes via contamination-induced degradation. Finally, the damage threshold at the wall region under 0.6-ps pulses (type-I damage mechanism) is considerably lower than that of the control coating, even after taking into consideration the 3× field enhancement. In addition, since damage initiated at the region of maximum field enhancement (and not on the wall itself), we postulate that contamination enters within the silica layer through the wall region and synergistically contributes (with pre-existing defects) to damage initiation. Conversely, the damage threshold in the wall region under 10-ps excitation is approximately half that of the control coating. We attribute this effect to statistics. Since damage is originating at precursor defect locations, and the region of the tested material with maximum electric field is narrow, damage initiates at the precursor defects in the maximum electric-field region that otherwise would not damage if tested in a coating with no field enhancement as observed in the case for the millimeter-pitch pillar top and sole. Consequently, performing a more-detailed study should reveal that there are locations when damage initiates at even lower fluences than those reported in Table 1.
Our experimental results using 1740-lines/mm MLD gratings showed analogous observations, where the damage threshold at various pulse lengths are lower (often significantly lower, depending on the manufacturer of the grating) than those expected based on the field enhancement in the constituent silica layer. Further decline was reported following exposure of the gratings to the operational environment. Future work will further explore the relationship of process-induced contamination to laser-induced damage.
5. Conclusion
Damage initiation in MLD gratings occurs predominantly in the pillars, causing pillar sections to be removed, therefore eliminating key signatures regarding the origin and governing damage mechanisms. To address this issue, millimeter-scale structures were fabricated to enable damage behavior investigation of the individual grating structures. We found that the lowest damage threshold is in the wall region, and the damage morphology suggests that initiation is similar to that observed in hafnia/silica coatings involving type-I or type-II damage sites, depending on pulse length. The damage morphology in actual MLD gratings is consistent with the observations seen in the millimeter-scale structures and hafnia/silica coatings, suggesting two different damage-initiation mechanisms are involved and are associated with different pulse-duration regimes.
Funding
National Nuclear Security Administration (DE-NA0003856); New York State Energy Research and Development Authority; University of Rochester.
Acknowledgments
This report was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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